Whereas known manually shifted and automatic transmissions for motor vehicles have stepped drive ratios and therefore do not allow the internal combustion engine to be operated in the range of high efficiencies in every driving situation, this problem can be eliminated by an electromechanically power-splitting hybrid drive system. Hybrid drive systems of this kind are disclosed, for example, in German Published Patent Application No. 198 42 452 (Toyota hybrid system), German Published Patent Application No. 199 03 936 (Dual-E transmission) or German Published Patent Application No. 199 09 424 (SEL 120/3 transmission). All these drive systems possess, in addition to the internal combustion engine, two electric motors that constitute an electrical actuating gear drive. The internal combustion engine and the electric motors are coupled by way of a downstream mechanical transmission having planetary stages, in which transmission the drive power of the internal combustion engine is divided into two power components. Whereas the one power component is transferred mechanically, and thus at high efficiency, to the transmission output shaft and thus to the motor vehicle's wheels, the other power component is converted into electrical power by the one electric motor in generator mode, and fed back into the transmission by the other electric motor in motor mode.
In drive systems of this kind, an additional starter and generator can be dispensed with. The electric motors start the internal combustion engine and generate the electrical power necessary for an electrical system of the motor vehicle that encompasses an energy reservoir for the electrical power that is generated. Also possible, in addition to a hybrid mode in which both the internal combustion engine and the electric motors operate, is a boost mode and a purely electrical driving mode; in the latter, electrical power is taken from the energy reservoir.
In hybrid mode, the decoupling of the rotation speeds of the two electric motors results in one rotation speed degree of freedom; this means that for a specified vehicle speed and therefore a specified rotation speed at the transmission output shaft (and for a specified gear ratio, in the case of the SEL or Dual-E transmission), the rotation speed of one of the two electric motors can be selected without restriction (within physical limits). The rotation speed of the second electric motor and the rotation speed of the internal combustion engine are then determined by the coupling conditions of the downstream transmission. This rotation speed degree of freedom is used to operate the drive train in the range of high efficiencies. A control system of the motor vehicle substantially takes into account its speed and the actual rotation speed of the transmission output shaft, as well as the mechanical power requested by the driver (accelerator pedal position) and the electrical power needed to supply the vehicle's electrical system, and on the basis of these parameters defines the rotation speed degree of freedom that is present, as well as the torques of the three drive units.
With the known methods for regulating a drive system of this kind having one rotation speed degree of freedom, one of the electric motors is operated in rotation-speed-regulated fashion, while the internal combustion engine and the other electric motor are torque-controlled, or the latter is torque-regulated in the case of an electric motor having a current regulator or field-oriented regulation system. In other words, a control system of the motor vehicle drive system specifies the target rotation speed of the rotation-speed-regulated electric motor, the target torque of the torque-controlled electric motor, and the target torque of the torque-controlled internal combustion engine. The two torque-controlled drive units influence not only the torque at the transmission output shaft but also the torque that occurs at the rotation-speed-regulated drive unit or is set there by a rotation speed controller of that unit, and is specified as the target value for its subordinate current regulation system. Ideally, this torque corresponds to a target torque, calculated in advance in the control system, for the rotation-speed-regulated electric motor.
With the known method, however, inaccuracies present especially in the torque control actions in the internal combustion engine, and inaccuracies in the friction conditions of the transmission, have an effect on the rotation-speed-regulated electric motor, with the result that the torque established at that electric motor by the rotation speed controller can deviate considerably from the target torque calculated in advance in the control system.
Certain negative effects result therefrom. On the one hand, in such a case the electrical power of the rotation-speed-regulated electric motor also deviates from the target value. The electrical power fed into the electrical system then does not correspond to the control system's specification, thus negatively affecting the electrical system. In addition, the power limits of the electrical energy reservoir can also be exceeded, e.g. in the context of energy recovery during a braking operation or in boost mode. On the other hand, inaccuracies can cause the rotation-speed-regulated electric motor to arrive at its maximum torque limit, which is equivalent to a limitation of the manipulated variable for the rotation speed control loop. The link established with the rotation speed regulation system thereby becomes ineffective. Without further interventions on the torque-controlled second electric motor or the torque-controlled internal combustion engine, control of the system becomes lost.
In dynamic mode, the torque-controlled drive units can be pilot-controlled based on a knowledge of the inertias that need to be compensated for. Additional inaccuracies may nevertheless be expected in this context, with effects in turn on the torque of the rotation-speed-regulated electric motor.